In telephone transmission circuits a certain amount of loop or line current is needed to properly operate a subscriber's central office line equipment. Loop current is typically the current flowing in a subscriber's loop which comprises a two conductor cable that connects the subscriber's telephone to the central office. In a given telephone circuit, the amount of loop current is limited by the resistance of the cable's conductors which is a function of cable length. Thus the maximum resistance which the cable presents to the central office limits the maximum distance a subscriber's telephone may be located from the central office equipment and still successfully operate that equipment. The normal loop resistance limit of a telephone central office is usually determined by the ring trip current which flows in the loop to trip the ring or the current required to pull in a relay connecting the subscriber's loop to a line finder at the central office.
In recent years a trend toward wider geographical distribution of a single telephone exchange has developed, thus necessitating subscriber loops which are longer than the normal loop resistance would allow. The use of one central office to provide service to a wide geographic area has allowed the cost of telephone service to surburban and rural areas to be minimized.
In the past, various methods have been employed in order to extend the allowable distance between a subscriber's telephone and the central office equipment. Arrangements whereby the voltage across the loop has been boosted have been used but they are usually somewhat complex and expensive to manufacture and also increase the currents drawn by shunt faults in the telephone cable.
Another method of drawing sufficient current to trip the ring or pull in a line finder relay has been to connect a shunt across the loop in order to increase the current drawn from the central office when a sensing device senses an off hook current present in the subscriber's loop. Such devices have drawbacks in their use, including the fact that maintenance of a shunt across the line after the ring is tripped or a line finder relay is pulled in also shunts the subscriber's telephone during its use and tends to degrade the signal level. Furthermore, such devices are prone to false ring trip in that they may be activated by the high voltage AC ringing signal present across the subscriber's loop when the telephone is ringing.